**Core Concept**
Cyanosis is a clinical sign characterized by a bluish discoloration of the skin and mucous membranes, typically resulting from an increased concentration of reduced hemoglobin (>5g/dL) in the blood. This condition is usually associated with conditions that lead to a significant decrease in the oxygen-carrying capacity of the blood.

**Why the Correct Answer is Right**
The correct answer is related to the fact that severe anemia is characterized by a significant decrease in the concentration of hemoglobin in the blood. In anemia, the reduced hemoglobin is not sufficiently high to cause cyanosis. This is because the reduced hemoglobin is still able to be oxygenated in the lungs, and the decreased oxygen-carrying capacity of the blood is not sufficient to cause a noticeable bluish discoloration of the skin. The critical factor for the development of cyanosis is the concentration of reduced hemoglobin, which is typically above 5g/dL.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the underlying reason for the absence of cyanosis in severe anemia.
**Option B:** This option is incorrect because it is a statement about a different condition (polycythemia vera) and does not relate to the question about severe anemia.
**Option C:** This option is incorrect because it is a statement about a condition that is not relevant to the question about severe anemia.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that cyanosis is a clinical sign that is typically associated with conditions that lead to a significant increase in the concentration of reduced hemoglobin in the blood. This is why conditions like severe anemia, which are characterized by a significant decrease in hemoglobin concentration, do not typically present with cyanosis.

**Correct Answer:** D.